Conventionally, gases have been injected into a subterranean hydrocarbon-bearing formation via a well in fluid communication with the formation to displace hydrocarbons from the formation to a well from which hydrocarbons are produced. Several qualities of a gas dictate the efficiency, and therefore success, of any process utilizing the gas to displace hydrocarbons from a subterranean formation. Preferably, a gas which is injected into a subterranean hydrocarbon-bearing reservoir swells hydrocarbons upon contact thereby reducing the viscosity of the hydrocarbons and permitting effective displacement of such hydrocarbons to a producing well in fluid communication with the subterranean formation. In addition, the efficiency of the injected gas in displacing hydrocarbons from the subterranean formation will be increased if the interfacial tension between the gas and the hydrocarbons in the matrix of the subterranean formation is reduced. Further reduction of such interfacial tension may be evidenced by the ability of the gas to be miscible with hydrocarbons present in a subterranean formation. A gas may be miscible with hydrocarbons upon first contact in a subterranean formation or may develop miscibility upon multiple contact with hydrocarbons present in a subterranean formation. Further, efficiency of the injected gas may be dependent upon alteration of formation wettability by the gas or the characteristics of multiphase flow exhibited by the gas and formation hydrocarbons. The effect each of these qualities will have on mobilizing hydrocarbons in a subterranean formation will be dependent on the temperature and pressure of the formation as well as the composition of the gas and formation hydrocarbons.
Carbon dioxide, methane, natural gas, nitrogen, and mixtures thereof have been previously utilized as an injection gas to displace hydrocarbons from a subterranean hydrocarbon-bearing formation. While such gases are economically attractive, they are less soluble in formation hydrocarbons than normally gaseous intermediate hydrocarbons, in particular C.sub.2 -C.sub.4 paraffins, i.e., liquified petroleum gas (LPG), and accordingly, result in less efficient recovery of hydrocarbons from a subterranean formation. It has been proposed to enhance a process for recovering hydrocarbons from a subterranean formation utilizing primary drive gases, such as carbon dioxide, methane, and/or nitrogen, by enriching such gas with intermediate hydrocarbon gases prior to injecting the primary drive gas into the formation so as to improve the qualities of the primary drive gas as an enhanced oil recovery agent. Such addition of intermediate hydrocarbon gases to a primary drive gas is feasible where the value of incremental oil attributable to the process in which the primary gas drive is utilized is greater than the value of the intermediate hydrocarbons incorporated in the primary gas drive.
The cost of adding intermediate hydrocarbons to the primary drive gas often is high, resulting in efforts to minimize the amount of intermediate hydrocarbons added. Even when the quantity of intermediate hydrocarbons added is maintained at a minimum level required to improve the qualities of the primary drive gas, the cost of the flooding process may still be unacceptably high. The relative unavailability of intermediate hydrocarbon gas at a field location may also contribute to an unacceptable process cost. Accordingly, a need exits for an economical and efficient process for enriching a primary drive gas to be utilized in a process to displace hydrocarbons from a subterranean hydrocarbon-bearing formation.
Accordingly, it is an object of the present invention to provide an economical process for enriching a primary drive gas to be utilized in a process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation.
It is a further object of the present invention to provide a reliable and economical source of intermediate hydrocarbon gas for enrichment of a primary drive gas.
It is another object of the present invention to provide a drive gas which is enriched with intermediate hydrocarbon compounds and which is capable of efficiently displacing hydrocarbons from a subterranean formation.